It is common to join metallic frame members by welding these together but this creates the problem firstly that the weld must be very adequately cleaned up afterwards and it is also very time consuming.
Various press on connectors have been proposed and in fact used hitherto including plastic fittings which are insertable within the end of the pipe but these suffer from providing an excessive resiliency to the connected members or, where they are on show, the appearance is not always preferable.